| The implementation of "rare earth industry emission standards" requires theenterprises to treat wastewaters both for reducing emission and recovering valuablesubstances. This paper studies on the technologies for treating wastewaters of yttriumoxalate precipitation in refine factories and leach solution containing lowconcentration of rare earth and ammonium in ion adsorption rare earth deposits.Several treat technologies are respectively suggested to meet the demand of materialsrecovering and wastewater discharging.Yttrium oxalate precipitation wastewater produced from the manufacture of highpurity of yttrium oxides contains many valuable substances such as hydrochloric acid,oxalic acid and yttrium. We designed a method to recover all these substances andwater by simply adding yttrium chloride solution to precipitate the excess oxalatewith a mole proportion of Y3+to C2O42-between0.650and1.106. After filtrating outthe yttrium oxalate which can be used as crystal seed for the following precipitation,the resultant solution contains1.7-2mol/L of hydrochloric acid, less than0.04mol/Lof oxalate and few of excess yttrium chloride, which can be used as washing acid oras basic solution of preparing stripping acid in the extraction separation process ofyttrium. This method is also suitable for treating oxalate precipitation wastewaters ofother rare earth elements. Meanwhile, the yttrium chloride solution used to removeoxalate can also be replaced by yttrium carbonate. In this situation, the removal ratioof oxalic acid is up to85%with a lower resident concentration of both oxalate andacid at around0.02mol/L.Clay minerals are good adsorbent for rare earth as well as other heavy metal ionsbecause their rapid adsorption and high capacity. We taken clay minerals from thetailings of ion adsorption rare earth deposits and separated them into two parts bysimply sieving with a100-200-mesh sieve, the fine particle clay (FPC) and coarseparticle (CPC). Then, they were modified by NaCl or NaCl-HCl solution to form Na+type or Na+-H+type FPC and CPC. Their adsorption isotherms for rare earths insolutions with different pH values and different concentrations of ammonia were obtained and the adsorption capacities for rare earth in solution with pH over5andammonia concentration less then50mg/L were calculated to be1.5-4.0mg/g for FPCand0.1-1.2mg/g for CPC, and decreased with the increase of ammonia concentrationand the decrease of solution pH. With FPC and CPC, we proposed different methodsto treat the leach solutions from the tailings of ion adsorption rare earth deposits. Forthe leach solution with concentration of ammonia and rare earths higher than30and10mg/L, the suggested program includes an adsorption and precipitation process forrare earth by FPC at pH over8, a break point chlorination technology for ammoniaremoving, and a bipolar membrane electrolysis process for the remaining electrolytes.But for the leach solution which concentration of ammonia and rare earths lower then30and10mg/L, only an adsorption process by CPC is needed. To recover the rareearth adsorbed on FPC and CPC, NaCl-HCl solution (7.5%, pH1) is employed asdesorbent to leach rare earth adsorbed FPC or CPC with weight ratio of clay tosolution1:5. The recovery rate of rare earth is found to be98.31%, and the desorbedclay can be reused. The comprehensive experiment results showed that the treatedwastewater can reach emission Standards, i.e.: concentration of rare earth is below0.5mg/L, ammonia below15mg/L. Moreover, the desorbed rare earth can berecovered by precipitation method, and the enriched ammonium or sodium chloridecan be transferred to ammonia or sodium hydroxide and hydrochloride acid by abipolar membrane electrolysis process. |
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